#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define endl '\n'
const int N = 2e5 + 5;

mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());
int n, m;
int a[N];
void solve() {
  cin >> n;
  rep(i, 1, n) cin >> a[i];
  sort(a + 1, a + n + 1);
  int mx = 0;
  for (int i = 1; i <= n; i += 2) {
    mx = max(mx, a[i + 1] - a[i]);
  }
  cout << mx << endl;
}
int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  int t;
  cin >> t;
  while (t--) solve();
  return 0;
}